I have an old Lap steel that has, rather than a pot for tone control, a switch that inserts the capacitor in the circuit. When it is switched in, there is a loud hum. Out, it works. Any ideas? Just wire in a pot? Can I test the cap?

Make sure the cap is wired in correctly, it goes between the hot lead and ground. If that's right then try a new cap....it doesn't have to be permanent, just to test things. If that fixes it, then find a nice one like an orange drop or something to replace the bad one.